1. Field of the Invention
This invention relates to a safety means for such heating appliances having a digital control section as electric ovens, microwave ovens, and composite ovens which comprise the two preceding types, wherein the preheating completion time is estimated and reported, and the completion of preparation for cooking is detected by the opening and closing of the door, the time for pushing the start switch is reported, and the completion of preparation is reported upon ascertainment of the completion of a timer setting.
2. Description of the Prior Art
The recent development of semiconductor technology is remarkable, and the incorporation of electronic circuits in control systems is increasing rapidly. With the advent of stored logic type controllers represented by the microcomputer, digitalization of control systems is also increasing rapidly.
In the field of heating appliances also, the wave of such digitalization has changed the arrangement of control systems and the configuration of operating panels to a large extent. New functions which would be impossible to attain with conventional discrete circuits have been added one after another.
However, such digitalization of operating panels and adaptation of heating appliances for multifunction use can complicate the operation of the appliance, resulting in an increasing tendency for the user to experience troubles; user fails to fully utilize the "helpful"functions or erroneously operates the appliance, leading to unsatisfactory heating. Even a failure of part of the circuitry can cause the control system to run away, contrary to the user's intention.
Accordingly, there has been a desire for a heating appliance which will never start heating until the user has set desired data on heating, will report to the user the acceptance of the effective heating data by the control system to thereby make the operation readily understandable, and is capable of preventing the control system from running away even in the event of a failure.
As for an error often found particularly in the composite oven, the user forgets to do the idle heating, which is called preheating, (i.e., heating with no load put in the heating chamber) and consequently fails in cooking or by mistake the idle heating is performed for a cooking mode which forbids it. The number of such errors is increasing.
The manner of operating the heating appliance largely depends on differences in the type of heat source and in the method of heating food.
First, reference will be given to heating appliances equipped with an electric heater or gas burner as a heat source, such as an oven (hot oven) and grill. These heat sources heat food externally. Further, these heating methods can be roughly classified into two types by the heating mechanism, as follows.
One type is oven heating wherein food is baked in a heated atmosphere. This is a heating method wherein food is baked in a high temperature atmosphere with the radiant heat from the heat source being prevented from reaching the food. In this case, it is necessary to sufficiently elevate the temperature in the heating chamber prior to putting food therein. That is, idle heating called preheating must be done. Neglecting this would require a longer heating time or even cause a failure in cooking depending upon the kind of dish.
The other type is grill heating wherein food is baked by the radiant heat from the heat source. This type does not require preheating since heating is carried out by the infrared radiation from the heat source. If preheating is performed by mistake, the interior of the heating chamber will reach a considerably high temperature in a short time , entailing danger. The user will be given a rough estimate of the required heating time for grill heating under conditions not including unnecessary preheating; thus, preheating would result in scorching.
While the foregoing refers to the difference based on the presence or absence of preheating, there is another great difference in operation between oven heating and grill heating. It lies in restart after the door is opened in the course of heating. Generally, the heat source used for oven heating is a sheathed heater which emits less radiation, so that the heater itself takes some time in raising the temperature in the heating chamber. Thus, it is often so arranged that once heating is started, electric supply to the sheathed heater is continued even if the door is opened in between. Of course, temperature control is performed. In such a case, therefore, oven heating is restarted simply by the user closing the door again.
On the other hand, a quartz tube heater and special sheathed heater are used for the grill heating and emit infrared radiation at high rates. These heaters exhibit a satisfactory rise in infrared radiation. There is no need to maintain the interior of the heating chamber at a high temperature. Thus, if the door is opened during heating, the electric supply to the heater will be cut off to protect the user against a burn. Thus, it is usually arranged so that the start key must be pressed again for a restart.
Reference will now be given to such heating appliances as a microwave oven, equipped with a microwave generator, e.g., a magnetron. Microwave heating heats an object internally and externally at the same time and needs no preheating. If, therefore, preheating (i.e., idle heating) is effected by mistake, though there is no danger of scorching as in grill heating this can be very dangerous since it results in an increased leakage of microwaves from the main body and in overheating local portions of the heating chamber (where the standing waves are strong). Further, it damages the magnetron, shortening its life. In other words, idle heating in the microwave oven is strictly prohibited.
For prevention of idle heating, the microwave oven must be so arranged that the start key must be pressed before heating can be restarted, each time the door is closed.
As described above, the manner of operation widely differs according to differences in heat source and heating method. Moreover, composite heating appliances in which these oven, grill and microwave heating methods are integrated are coming to have a share of the greater part of the demand. Thus, the user should accurately remember and carry out the disparate operations for the respective heating methods. A mistake in these operations entails not only a failure in cooking but also possible dangers of hurting the person and even causing a fire.
Thus, preheating has been a difficult thing, particularly with respect to the composite oven, as described above. Moreover, preheating has the following problems left unsolved.
How to carry out preheating--there are three methods, roughly classified. A first method is to set a necessary preheating temperature and wait until heating to this temperature. This is the intrinsic form of preheating, superior in the functional aspect of elevating the temperature in the heating chamber to a predetermined value, and is capable of completing preheating in a shorter time than any of the other methods which follow, though it dose not allow the user to leave the heating appliance since how long preheating will take is unknown.
A second method is to set a heating time alone and continue supplying electricity to the heater or the like serving as the heating means until a predetermined period of time has elapsed. The time at which preheating ends is obvious, so that this method is the most simple and convenient method for the user. However, since the preheating temperature is directly influenced by the heating chamber temperature and the source voltage at the start of preheating, different preheating temperatures will result; thus, this method is inferior in a functional aspect.
A third method, which is a combination of the preceding two methods, is to set both a preheating temperature and a preheating time. According to this method, after the preheating temperature is reached, temperature control is performed until the present time has elapsed; thus, this method is superior both in a functional aspect and in a operating aspect in that the preheating completion time is made known. As compared with the first method, however, it requires extra time and energy. The operating procedure is also increased, imposing an increased burden on the user.